This invention relates generally to the fabrication of film bulk acoustic resonator filters.
Radio frequency front-end filters may include film bulk acoustic resonators (FBARs) having many advantages compared to other technologies such as surface acoustic wave (SAW) devices and ceramic filters, particularly at relatively high frequencies. For example, SAW filters start to have excessive insertion loss above 2.4 gigahertz and ceramic filters are much larger in size and become increasingly difficult to fabricate as frequency increases.
Film bulk acoustic resonators include an upper electrode and a lower electrode that sandwich a piezoelectric film such as ZnO or AlN. The material properties of these films, including their longitudinal piezoelectric coefficient and acoustic loss coefficient, are the key parameters for the resonator's performance, such as the electric/mechanical coupling, and quality (Q) factor. High quality factors of film bulk acoustic resonators improve their insertion loss and pass-to-stop band roll-off. High electromechanical coupling of film bulk acoustic resonators improves the bandwidth of the filter.
Thus, there is a need for better ways to improve the properties of film bulk acoustic resonators.